Large systems can be controllably entangled and limitlessly measured

Quantum mechanics governs both fundamental particles and large objects. But at the macroscale, quantum effects are all but invisible because classical effects dominate. In recent experiments based on pairs of microns-thick drumhead membranes that oscillate together in near-perfect synchronization, two research groups now report direct observations of quantum mechanical effects. The ability to manipulate those effects in macroscale devices could lead to new designs for logic gates and enhanced measurement techniques.

In one paper, Shlomi Kotler, then at NIST, and colleagues directly measured quantum entanglement in a classical system. The NIST team started with two aluminum drumhead membranes that were separated by a resonant cavity. Microwave pulses directed at the membranes provided a force that steered them into a synchronized oscillating motion. When another pulse was reflected off the oscillating membranes, the reflected wave was Doppler shifted by their motion and thus contained direct information about each membrane’s position and momentum. By repeating the experiment 10 000 times and charting the evolution of the membranes’ movements, the researchers found that the instantaneous positions and momenta tracked one another with precision beyond the threshold permitted by classical physics. That perfect correlation is a hallmark of quantum entanglement.

In the other paper, Laure Mercier de Lépinay and colleagues at Aalto University in Finland created a system in which they could measure a vibrating membrane’s position and momentum while avoiding quantum back action. They developed a single effective oscillator composed of two separate microwave cavities housing two vibrating membranes, whose motion was driven collectively by microwave beams. By treating the two membranes as a single entity and measuring its position and momentum, the researchers crafted a method to ensure that the uncertainty associated with each membrane canceled out or was hidden in a part of the system that wasn’t directly observed. Instead, observing the resonant frequency of the single effective oscillator allowed for complete measurement of the system without its overall state being disturbed through quantum back action.

Kotler’s pulse-driven entangled system could be scaled to multiple logic gates for quantum information processing, and Mercier de Lépinay’s measurement technique could be the basis of sensors that exceed the sensitivity of their classical counterparts. The vibrating membrane platform can be easily manufactured and provides a tool for exploring the limits of quantum phenomena. (S. Kotler et al., Science 372, 622, 2021 ; L. Mercier de Lépinay et al., Science 372, 625, 2021 .)